Method for the removal of nox and n2o from the residual gas in nitric acid production

ABSTRACT

A process is described for reducing the NO x  concentration and N 2 O concentration from the residual gas from nitric acid production. The process encompasses the passing of the residual gas leaving the absorption column, prior to entry into the residual gas turbine, through a combination of two stages. The first stage here reduces the NO x  content and the second stage the N 2 O content of the gas, the NO x /N 2 O ratio prior to entry of the gas into the second stage being in the range from 0.001 to 0.5, and this gas being brought into contact in the second stage with a catalyst which is substantially composed of one or more iron-loaded zeolites.

[0001] The present invention relates to a process for eliminating NO_(x) and N₂O from the residual gas from nitric acid production.

[0002] Industrial production of nitric acid HNO₃ by catalytic combustion of ammonia produces a waste gas loaded with nitrogen monoxide NO, nitrogen dioxide NO₂ (together termed NO_(x)), and also nitrous oxide N₂O. While NO and NO₂ have long been recognized as compounds having relevance to environmental toxicity issues (acid rain, smog formation), worldwide limits having been set for maximum permissible emissions of these materials, the focus of environmental protection has in recent years increasingly also been directed toward nitrous oxide, since it makes a not inconsiderable contribution to the decomposition of stratospheric ozone and to the green-house effect.

[0003] After the adipic acid industry has reduced emissions of nitrous oxide, nitric acid production is the largest source of industrial emissions of nitrous oxide. For reasons of environmental protection, therefore, there is an urgent requirement for technical solutions for reducing nitrous oxide emissions as well as NOx emissions during nitric acid production.

[0004] There are numerous versions of processes for eliminating NOx from the waste gas from nitric acid production (termed here the DeNOx stage), such as chemical scrubbing, adsorption processes, or catalytic reduction processes. Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 17, VCH Weinheim (1991) (D1) gives an overview. Emphasis should be given here to selective catalytic reduction (SCR) of NOx by means of ammonia to give N₂ and H₂O. Depending on the catalyst, this reduction can proceed at temperatures of from about 150° C. to about 450° C., and permits more than 90% NO_(x) decomposition. This is the version of NOx reduction mostly utilized during nitric acid production, but, like the other versions, does not lead to any reduction in N₂O content.

[0005] For this purpose the current prior art requires a separate, second catalyst stage, advantageously combined with the DeNOx stage.

[0006] An example of the process based on this approach is described in U.S. Pat. No. 5,200,162, which claims the decomposition of N₂O in a waste gas which also comprises NOx, downstream of a DeNOx stage. Here, at least one substream of the waste gas which leaves the N₂O decomposition stage is cooled and returned thereto in order to avoid overheating of this stage due to the exothermic nature of the N₂O decomposition process. The invention relates to waste gases whose N₂O content is up to 35% by volume, e.g. to waste gases from adipic acid production.

[0007] A process put forward by Shell describes the integrated elimination of NOx and N₂O in the residual gas from nitric acid production (Clark, D. M.; Maaskant, O. L.; Crocker, M., The Shell DeNOx System: A novel and cost effective NOx removal technology as applied in nitric acid manufacture and associated processes, presented at Nitrogen '97, in Geneva, Feb. 9-11, 1997, (D2)).

[0008] The Shell reactor system is based on what is called a lateral flow reactor principle, where even relatively low temperatures (from 120° C.) are possible for the operation of the DeNOx stage. An amorphous metal oxide catalyst is used for removing N₂O.

[0009] When appropriate catalysts are arranged in the residual gas leaving the absorption column with a temperature of from 20 to 30° C., the latitude for possible operating temperatures is prescribed by the operating temperature of the residual gas turbine.

[0010] Specifically, for reasons associated with the technical and economic running of the entire process, the residual gas turbine should most advantageously be operated with entry temperatures <550° C. and with maximum ΔT and Δp.

[0011] This is particularly important for eliminating N₂O, since according to current prior art this requires markedly higher temperatures than those needed during catalytic reduction of NOx. The cost-effectiveness of this option is therefore linked to adequate catalyst activity.

[0012] Kapteijn F.; Rodriguez-Mirasol, J.; Moulijn, J. A., Appl. Cat. B: Environmental 9 (1996) 25-64, (D3) gives an overview of the numerous catalysts which have been demonstrated to be suitable in principle for decomposing and reducing nitrous oxide. Metal-exchanged zeolite catalysts (U.S. Pat. No. 5,171,533), inter alia, appear particularly suitable for decomposing N₂O.

[0013] The zeolites used here are prepared by ion exchange in an aqueous solution comprising metal salts. The metals used for the ion exchange are from the group: copper, cobalt, rhodium, iridium, ruthenium, and palladium. The copper zeolites are highly sensitive to water vapor and rapidly lose their activity under those conditions (M.; Sandoval, V. H.; Schwieger, W.; Tissler, A.; Turek, T.; Chemie Ingenieur Technik 70 (1998) 878-882, (D5)), while the other metals listed here are relatively expensive.

[0014] Using iron-doped zeolite of Fe-ZSM5 type under appropriate conditions, as described in Table 1 in U.S. Pat. No. 5,171,533, in the absence of NOx, H₂O, and O₂ at 450° C., only 20% N₂O decomposition was achieved.

[0015] The activity of Fe-ZSM-5 for decomposing N₂O is, however, markedly increased in the presence of appropriate amounts of NO, this being attributed to a reaction forming NO₂ as in NO+N₂O→N₂+NO₂, catalyzed by Fe-ZSM-5 (Kapteijn F.; Marban, G.; Rodrigeuez-Mirasol, J.; Moulijn, J. A., Journal of Catalysis 167 (1997) 256-265, (D6); Kapteijn F.; Mul, G.; Marban, G.; Rodrigeuez-Mirasol, J.; Moulijn, J. A., Studies in Surface Science and Catalysis 101 (1996) 641-650, (D7)).

[0016] In the absence of NOx, higher activity was found for Cu or Co-exchanged zeolites than for the corresponding Fe zeolites.

[0017] In the descriptions set out in the prior art (D6, D7) of N₂O decomposition in the presence of an Fe-ZSM-5-catalyst at 400° C., use is usually made of equimolar amounts of NO and N₂O. In D6 and D7, the effect of NOx on N₂O decomposition falls constantly as NO/N₂O ratio sinks, and therefore N₂O decomposition becomes unsatisfactory when the NO/N₂O ratio is below 0.5.

[0018] The best results are found when the molar ratio NO/N₂O is 1 or greater than 1.

[0019] According to the authors, when this catalyst is used for N₂O reduction in the waste gas from nitric acid production, the NO₂ formed could be returned to the process for obtaining HNO₃. Depending on the version of the process, the NOx concentration and N₂O concentration in the waste gas are about 1 000 ppm.

[0020] WO 99/34901 relates to iron-containing zeolites based on ferrierite for reducing N₂O-containing gases. The catalysts used here comprise from 80 to 90% of ferrierite, and also binders. The water content of the gases to be reduced is in the range from 0.5 to 5%. When various zeolite types are compared, the best results for decomposition of N₂O at temperatures of from 375 to 400° C. were obtained using zeolites of FER (ferrierite) type (97% N₂O decomposition at 375° C. and NO/N₂O=1). Substantially less decomposition was found when using zeolites of pentasil (MFI) type or mordenite (MOR) type. Indeed, the maximum N₂O decomposition achievable under the above conditions when iron-containing MFI zeolites were used was only 62%.

[0021] In the light of the known prior art, it is therefore an object to provide an economic process, in particular for HNO₃ production, which permits not only high levels of NOx decomposition but also satisfactory N₂O decomposition.

[0022] In particular, good results for N₂O decomposition are to be obtained even when the NOx/N₂O ratio is sub-stoichiometric, in particular at the ratios which result after NOx content reduction, i.e. at a ratio <0.5, preferably <0.1.

[0023] The present invention achieves this object and provides a process for reducing the NO_(x) concentration and N₂O concentration from the residual gas from nitric acid production, where the residual gas leaving the absorption column is passed, prior to entry into the residual gas turbine, through a combination of two stages, the first stage (DeNOx stage) reducing the NOx content and the second stage (DeN₂O stage) reducing the N₂O content of the gas, and where the NOx/N₂O ratio prior to entry starting materials here are the commercially available ammonium zeolites (e.g. NH₄-ZSM-5) and the appropriate iron salts (e.g. FeSO₄×7 H₂O), these being mixed intensively with one another by mechanical means in a bead mill at room temperature. (Turek et al.; Appl. Catal. 184, (1999) 249-256; EP-A-0 955 080). These citations are expressly incorporated herein by way of reference. The resultant catalyst powders are then calcined in a furnace in air at temperatures in the range from 400 to 600° C. After the calcination process, the Fe zeolites are thoroughly washed in distilled water, and the zeolites are filtered off and dried. The resultant Fe zeolites are finally treated with the suitable binders and mixed, and extruded to give, for example, cylindrical catalyst bodies. Suitable binders are any of the binders usually used, the most commonly used here being aluminum silicates, e.g. kaolin.

[0024] According to the present invention, the zeolites which may be used are iron-loaded zeolites. The iron content here, based on the weight of zeolite, may be up to 25%, but preferably from 0.1 to 10%. Particularly suitable zeolites here are of the type MFI, BETA, FER, MOR, and/or MEL. Precise details concerning the build or structure of these zeolites are given in the Atlas of Zeolithe Structure Types, Elsevier, 4th revised Edition 1996, which is expressly incorporated herein by way of reference. According to the invention, preferred zeolites are of MFI (pentasil) type or MOR (mordenite) type. Zeolite Fe-ZSM-5 type are particularly preferred.

[0025] According to the present invention, DeN₂O catalysts are arranged in combination with an upstream DeNOx stage, between the absorption column and the residual gas turbine, in such a way that the residual gas leaving the absorption column is first passed at temperatures <400° C., in particular <350° C., into a reactor (first stage) in which the NOx content of the gas is reduced to, for example, <100 ppm (cf. FIG. 2). The operating pressure for this first stage is preferably from 1 to 15 bar, in particular from 4 to 12 bar.

[0026] The upstream DeNOx stage corresponds to a process usually used in nitric acid plants according to the prior art for reducing the amount of NOx emissions. However, the remaining NOx content of the residual gas has to be sufficiently high to permit the cocatalytic effects of NO or NO₂ to be active in the downstream DeN₂O stage.

[0027] If the DeN₂O stage is operated without upstream DeNOx, i.e. if the entering stream has approximately equimolar amounts of NO and N₂O, return of the NO₂ formed by NO+N₂O→N₂+NO₂ into the HNO₃ process is uneconomic, due to the relatively low NO₂ concentration, <2000 ppm.

[0028] The N₂O content of the gas remains substantially unaltered in the DeNOx stage. After leaving the first stage, therefore, the NO_(x) content of the gas is usually from 1 to 200 ppm, preferably from 1 to 100 ppm, in particular from 1 to 50 ppm, and its N₂O content is from 200 to 2 000 ppm, preferably from 500 to 1 500 ppm. The resultant NOx/N₂O ratio after leaving the DeNOx stage is from 0.001 to 0.5, preferably from 0.001 to 0.2, in particular from 0.01 to 0.1. The water content of the gas, both after leaving the absorption column and, respectively, the DeNOx stage and after leaving the DeN₂O stage, is usually in the range from 0.05 to 1%, preferably in the range from 0.1 to 0.8%, in particular in the range from 0.1 to 0.5%.

[0029] The residual gas conditioned in this way is then passed into the downstream DeN₂O stage, where decomposition of the N₂O into N₂ and O₂ is brought about by utilizing a cocatalytic effect of NOx in the presence of the appropriate zeolite catalyst.

[0030] Surprisingly, it was found that in the presence of the iron-containing zeolite catalysts used according to the invention N₂O decomposition is drastically increased (cf. FIG. 1) even in the presence of small amounts of NO_(x), i.e. when the molar NO_(x)/N₂O ratio is <0.5. An effect which becomes markedly more pronounced as the temperature increases. According to the present invention, therefore, at 450° C., for example, a molar NO_(x)/N₂O ratio of 0.01 is still sufficient to lower the N₂O concentration from 72% to 33% in the presence of an Fe-ZSM-5 catalyst. This is all the more astounding since in the prior art the accelerated decomposition of N₂O is attributed to the abovementioned stoichiometric reaction of N₂O with NO. If which become unsatisfactory (cf. Example 5). The content of N₂O in the process of the invention after leaving the DeN₂O stage is in the range from 0 to 200 ppm, preferably in the range from 0 to 100 ppm, in particular in the range from 0 to 50 ppm.

[0031] The operating temperature for the DeN₂O stage here is in particular determined by the desired degree of decomposition of N₂O and the amount of NO_(x) present in the residual gas, but also, as is known to the skilled worker and like almost all catalytic waste gas purification processes, highly dependent on the catalyst loading, i.e. on the waste gas throughput based on the amount of catalyst. The operating temperature for the second stage is preferably in the range from 300 to 550° C., in particular in the range from 350 to 500° C., the pressure being in the range from 1 to 15 bar, in particular from 4 to 12 bar. As pressure rises, the cocatalytic action of NO_(x) on N₂O decomposition becomes greater, and increase of pressure therefore permits a further drop in operating temperature.

[0032] In determining or setting the operating temperature, account also has to be taken of the content of oxygen and H₂O. This content can vary within certain limits, depending on the mode of operation and on the version of the process used for nitric acid production, and inhibits N₂O conversion. The O₂ content is in the range from 1 to 5% by volume, in particular in the range from 1.5 to 4% by volume.

[0033] N₂O decomposition of >90%, in particular >95%, can therefore be achieved at temperatures in the range from 300 to 550° C., preferably from 350 to 500° C., using the iron-containing zeolite catalysts used according to the invention. As temperature rises it is even possible to achieve satisfactory N₂O decomposition when the NO_(x)/N₂O ratio is 0.01.

[0034] By combining a DeNOx stage and a DeN₂O stage, the process of the invention permits the NO_(x) content and N₂O content of the residual gas to be reduced to minimal values during nitric acid production. The arrangement of the DeNOx stage prior to the DeN₂O stage and between absorption column and residual gas turbine moreover makes the process of the invention very economic, due to the continuously rising temperature profile.

[0035] Furthermore, the arrangement of both stages prior to the decompression turbine makes the conduct of the process particularly advantageous, since both stages can be operated at superatmospheric pressure (between 4 and 11 bar, depending on the version of the HNO₃ process), resulting in a reduction of the volume of reactor and, respectively, catalyst effectively needed.

[0036] Furthermore, since the DeNOx stage operates even at relatively low temperatures, sufficient reduction of NO_(x) content during plant start-up is also ensured when only little process heat is available.

[0037] Another advantage of arranging both stages between absorption column and residual gas turbine in a continuously rising temperature profile is that the residual gas leaving the inventive combination can be introduced, without prior cooling, and without any other measures for waste gas purification, directly to the residual gas turbine for ideal reclamation of compressive and thermal energy.

EXAMPLES

[0038] DeNOx Stage:

[0039] The DeNOx catalyst used as described with NH₃ as reducing agent upstream of the DeN₂O catalyst was a conventional SCR catalyst based on V₂O₅—WO₃—/TiO₂ (cf., for example, G. Ertl, H. Knözinger J. Weitkamp: Handbook of Heterogeneous Catalysis, Volume 4, pages 1633-1668). This was operated at a temperature of 350° C. Depending on the amount of NH₃ introduced, various NO_(x) contents and therefore NOx/N₂O ratios were set at the outlet from the DeNOx stage.

[0040] DeN₂O Stage:

[0041] An iron-containing MFI catalyst was prepared by solid-phase ion exchange, starting from a commercially available ammonium-form zeolite (ALSI-PENTA, SM27). Detailed information concerning the preparation process may be obtained from: M. Rauscher, K. Kesore, R. Mönnig, W. Schwieger, A. Tiβler, T. Turek, Appl. Catal. 184 (1999) 249-256.

[0042] The catalyst powders were calcined in air for 6 h at 823 K, washed and dried overnight at 383 K. Extrusion to give cylindrical catalyst bodies (2×2 mm) followed after addition of appropriate binders.

[0043] The experiments were carried out in a flux apparatus operated at steady state with on-line analysis, the space velocity in each case being 10 000 h⁻¹. The composition of the feed was: 1 000 ppm NOx

[0044] 1 000 ppm NO_(x)

[0045] 1 000 ppm N₂O

[0046] 0.5% vol H₂O

[0047] 2.5% vol O₂

[0048] remainder N₂

[0049] The following residual concentrations of NO_(x) and N₂O were obtained by varying the amount of NH₃ added: Resultant NO_(x) Resultant Resultant N₂O concentration NO_(x)/N₂O concentration Ex- (after DeNOx ratio (after (after DeN₂O am- Amount of stage at DeNOx stage at ple NH₃ added 350° C. stage) 475° C.) 1 500 ppm 500 ppm  0.5 40 ppm 2 800 ppm 200 ppm  0.2 54 ppm 3 950 ppm 50 ppm 0.05 81 ppm 4 990 ppm 10 ppm 0.01 99 ppm 5 1000 ppm  <1 ppm <0.001 462 ppm 

[0050] As can be seen from the examples given, a high level of N₂O decomposition is possible up to an NO_(x)/N₂O ratio of 0.001, in particular 0.01. If the ratio sinks below limit, the decomposition level becomes inadequate, there is no longer sufficient cocatalytic ion of NO_(x). 

What is claimed is:
 1. A process for reducing the NO_(x) concentration and N₂O concentration from the residual gas from nitric acid production, where the residual gas leaving the absorption column is passed, prior to entry into the residual gas turbine, through a combination of two stages, the first stage reducing the NO_(x) content by catalytic reduction, and the second stage reducing the N₂O content of the gas by decomposition into nitrogen and oxygen, and where the molar NO_(x)/N₂O ratio prior to entry of the gas into the second stage is in the range from 0.01 to 0.5, and in the second stage this gas is brought into contact with a catalyst which comprises one or more iron-loaded zeolites, the operating pressure in the second stage being from 4 to 12 bar.
 2. The process as claimed in claim 1, characterized in that the iron-loaded zeolite(s) present in the catalyst are of MFI, BEA, FER, MOR and/or MEL type.
 3. The process as claimed in claim 2, characterized in that the iron-loaded zeolite(s) are of MFI type.
 4. The process as claimed in claim 3, characterized in that the zeolite is an Fe-ZSM-5.
 5. The process as claimed in at least one of the preceding claims, characterized in that the temperature of the first stage is <400° C., preferably <350° C.
 6. The process as claimed in at least one of the preceding claims, characterized in that the temperature of the second stage is in the range of 300 and 550° C., preferably in the range of 350 and 500° C.
 7. The process as claimed in at least one of the preceding claims, characterized in that both stages are operated at a pressure in the range of from 4 to 12 bar.
 8. The process as claimed in at least one of the preceding claims, characterized in that the first stage is operated using the SCR process.
 9. The process as claimed in at least one of the preceding claims, characterized in that, after leaving the absorption column and prior to entry into the first or second stage, use is made of a residual gas whose water content is in the range from 0.05 to 1% by volume, in particular in the range from 0.1 to 0.8% by volume.
 10. The process as claimed in at least one of the preceding claims, characterized in that, prior to entry into the second stage, use is made of a residual gas whose NO_(x) content is in the range from 1 to 200 ppm and whose N₂O content is in the range from 200 to 2 000 ppm. 